Jan
12, 2005 ó Just as NOAA Weather Service
meteorologists exhale after a very grueling two weeks of severe winter
weather in the western and central United States, NOAA scientists have
identified one of the leading causes. "A leading climate culprit
is the Madden-Julian Oscillation or MJO,"
said Wayne Higgins, lead climate specialist at the NOAA
Climate Prediction Center located in Camp Springs, Md. (Click
NOAA image for larger view of weather patterns known as the “pineapple
express" where a significant amount of the deep tropical moisture
traverses the Hawaiian Islands on its way towards the western United
States. Click here for high
resolution version, which is a large file. Please credit “NOAA.”)

The MJO
is a tropical disturbance that influences the patterns of tropical rainfall
on time scales of approximately 30-60 days. It produces El
Niño-like features that can influence extreme precipitation
events in the western United States. Typically, the MJO is most active
during El Niño-neutral and weak-El Niño winters.

"Presently,
we are in a weak warm-episode (El Niño)," said Vernon Kousky,
NOAA's lead El Niño/La Niña scientist. He added, "El
Niño conditions are evident in the tropical Pacific and these
conditions are expected to continue for the next three months."

During
the last half of December, the MJO strengthened as enhanced cloudiness
and precipitation over the Indian Ocean shifted eastward. (This
activity was not related to the tsunami that occurred late in December.)
By early January 2005, the precipitation associated with the MJO extended
into the western tropical Pacific.

"As
the tropical rainfall associated with the MJO shifted eastward towards
the Central tropical Pacific, the jet stream over the North Pacific
gradually shifted eastward towards the California coast," said
Higgins. "This allowed recent storms to tap a deep-tropical moisture
stream that dramatically increased the precipitation over California,"
Higgins added.

The typical
scenario linking the pattern of tropical rainfall associated with the
MJO to extreme precipitation events along the West Coast of the United
States is shown in a NOAA
graphic. This graphic depicts changes in large-scale circulation
and moisture patterns that occur over the 10-day period leading up to
heavy precipitation events along the West Coast of the United States.
"The scenario shown in the schematic is exactly what we observed
over the past two weeks," said Higgins. These events can bring
up to several days of heavy rain and possible flooding. They are often
referred to as "pineapple
express" events, so named because a significant amount of the
deep tropical moisture traverses the Hawaiian Islands on its way towards
the western United States.

NOAA meteorologists
use a variety of data and analysis techniques to detect the MJO. Of
primary importance is information derived from NOAA's
polar-orbiting and geostationary satellites, which are used to identify
regions of heavy tropical precipitation. A second fundamental data source
is the global radiosonde network, which provides crucial information
regarding the atmospheric winds, temperature, moisture and pressure
fields at many levels of the atmosphere. These data are taken twice
daily and used in dynamical forecast models for weather and climate
prediction.

Yet, due
to its slowly evolving nature, accurate prediction of the MJO is difficult
beyond a week or two. "Good forecasts are related to our ability
to monitor the feature and to determine its relative position and strength,"
Higgins said. Dynamical models generally do not predict the MJO well
because of difficulties that remain regarding the correct mathematical
treatment of tropical rainfall processes. Because of this, it is difficult
to predict whether the MJO will remain active during the remainder of
the winter and what influences this might have on USA weather patterns.
"It is important to emphasize that these are relatively rare events
and that not all MJO events will lead to precipitation extremes in the
western United States," said Higgins.

Though
the MJO is a naturally occurring component of the coupled ocean-atmosphere
system, the basic reasons that explain why it is active in some seasons
and not in others are elusive and are the subject of ongoing research.

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Climate Prediction Center, an arm of the NOAA Weather Service, serves
the public by assessing and forecasting the impacts of short-term climate
variability, emphasizing enhanced risks of weather-related extreme events,
for use in mitigating losses and maximizing economic gains.

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